Last Sunday, a tragic monorail crash at Walt Disney World (between the Pink and Purple trains) killed the driver of Purple, Austin Wuennenberg. In the aftermath, Disney closed the monorail system for investigation and there has been a surprising amount of web coverage of the incident from monorail experts and drivers. But one forum post caught my attention. Written by a former monorail driver (sorry, "railie"), the post is educated speculation about what might have happened. It's interesting mainly because the immense technical detail revealed about the monorail system -- including its safety features, and information about what drivers must do in order to override them. Below are a few snippets, after a YouTube slideshow that explains the crash in detail.

The system at Disney is called the MAPO system, or more precisely the Moving Blocklight System (MBS). It consists of a number of transmitters along the beam every 7-10 pylons or so that place RF signals of three different frequencies onto the positive buss bar (power rail), and a corresponding receiver in each train. The trains are wired with a capacitor that shorts the MAPO signals to ground, preventing any signals generated ahead of the train from getting past it. The transmitters are arranged sequentially around the beam- if any given transmitter is putting out frequency #1, then the next one will be emitting frequency #2, and the next one after that will have frequency #3. The one after that will be transmitting frequency #1 again, and the cycle continues all the way around the beam. The upshot of this is that in normal operation, the following distance should be such that there will be three or more transmitters between a given train and the train ahead of him, thus the following train will "see" all three frequencies, and the driver will have a green MBS light on his console.

...there are a number of situations where the MAPO system needs to be turned off, and for that, there's a "MAPO override" button on the console, which allows the driver to do just that. When MAPO override is active, the train is limited to 15 mph, and the driver has to continue to hold the button down to keep the system overridden. Some examples of when the system needs to be overridden are when trains are on any of the spurlines (since they have no MAPO transmitters), or when trains are being switched between beams.